# std::ranges::range_adaptor_closure

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< class D > requires std::is_object_v<D> && std::same_as<D, std::remove_cv_t<D>> class range_adaptor_closure {}; |  | (since C++23)

`std::ranges::range_adaptor_closure` is a helper class template for defining a RangeAdaptorClosureObject.

Let t be the object of type `T`, the implementation ensures that t is a range adaptor closure object if all the requirements are met:

- t is a unary function object that takes one `range` argument.
- `T` has exactly one public base class ranges::range_adaptor_closure<T>, and `T` has no base classes of type ranges::range_adaptor_closure<U> for any other type `U`.
- `T` does not satisfy `range`.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges` | `202202L` | (C++23) | `std::ranges::range_adaptor_closure`

### Example

Run this code

    #include <ranges>
    #include <string_view>

    // Define Slice as a range adaptor closure
    struct Slice : std::ranges::range_adaptor_closure<Slice>
    {
        std::size_t start = 0;
        std::size_t end = std::string_view::npos;

        constexpr std::string_view operator()(std::string_view sv) const
        {
            return sv.substr(start, end - start);
        }
    };

    int main()
    {
        constexpr std::string_view str = "01234567";

        constexpr Slice slicer{.start = 1, .end = 6};

        // use slicer as a normal function object
        constexpr auto sv1 = slicer(str);
        static_assert(sv1 == "12345");

        // use slicer as a range adaptor closure object
        constexpr auto sv2 = str | slicer;
        static_assert(sv2 == "12345");

        // range adaptor closures can be composed
        constexpr auto slice_and_drop = slicer | std::views::drop(2);
        static_assert(std::string_view(str | slice_and_drop) == "345");
    }

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